Propylene resins hold an important position as a general-purpose resin material in industrial fields because of their excellent performances such as moldability and various properties, profitability, and suitability for the mitigation of environmental issues. However, the resins are relatively low in impact resistance, which is an important property. Techniques for improving the impact resistance have hence been well known which employ a propylene/ethylene random copolymer or a composition obtained by blending polypropylene with the copolymer.
Of such compositions, a typical example of the compositions obtained through a series of polymerization steps is one obtained by producing crystalline polypropylene in a first step and producing a propylene/ethylene copolymer in a second step. This composition, which is usually called a propylene/ethylene block copolymer, exhibits excellent performances with respect to a balance between rigidity and impact resistance and, hence, has been used in many industrial fields including automotive interior/exterior trim parts.
Industrially, such propylene/ethylene block copolymers are mostly produced with a Ziegler catalyst. It is, however, known that a Ziegler catalyst generally has many kinds of active sites (so-called multisite) and brings about a wide molecular-weight distribution and a wide comonomer composition distribution in propylene/ethylene copolymer parts.
In such a propylene/ethylene copolymer having a wide composition distribution, propylene segments or ethylene segments may be present in the copolymer in such an amount that crystallization is possible. These segments are thought to adversely influence the impact resistance of the block copolymer. It has further been pointed out that low-ethylene-content components and low-molecular components among the components of the copolymer ingredient dissolve in the crystalline polypropylene ingredient, resulting in impaired heat resistance. (See patent document 1.)
Many attempts have hence been made to improve rigidity, heat resistance, and impact resistance while attaining a satisfactory balance among these by producing a propylene/ethylene block copolymer with a metallocene catalyst, which recently has come to hold an important industrial position. In all these attempts, a crystalline homopolymer of propylene or copolymer with a small amount of ethylene and a propylene/ethylene copolymer are produced by two-stage polymerization (see patent documents 1 to 4).
A great feature of metallocene catalysts resides in that the polymerization-active sites are even (single sites). Compared to Ziegler catalysts, metallocene catalysts give a narrower molecular-weight or comonomer composition distribution. Consequently, a more homogeneous and more flexible rubber ingredient is yielded to improve impact resistance. It is further thought that since compatibility with the crystalline polypropylene ingredient can also be regulated, the problem concerning heat resistance is mitigated.
However, to have a narrow composition distribution on the other hand means a reduced affinity between the crystalline polypropylene ingredient and the propylene/ethylene copolymer ingredient. This adversely influences the particle diameter and interfacial strength of the copolymer ingredient (elastomer ingredient). For the simple block copolymer production in which two ingredients, i.e., crystalline polypropylene and a copolymer ingredient, are produced by merely using a metallocene catalyst, it is currently difficult to obtain a propylene block copolymer which satisfy all of rigidity, heat resistance, impact resistance, and the like while attaining a satisfactory balance among these.
Incidentally, it is thought that for improving the rigidity, heat resistance, impact resistance, and the like of such a propylene block copolymer while well balancing these, it is necessary that sufficient impact resistance should be maintained with the copolymer ingredient and, simultaneously therewith, compatibility between the crystalline polypropylene and the copolymer ingredient should be regulated so as to be in a proper range. The problem concerning compatibility between a crystalline polypropylene ingredient and a copolymer ingredient has been encountered also in propylene/ethylene block copolymers produced hitherto with a Ziegler catalyst. A technique for enhancing compatibility between a polypropylene ingredient and copolymer parts has been known from long ago which comprises adding a compatibilizing agent ingredient for these (see patent documents 5 and 6). In this technique, a polypropylene ingredient, a propylene/ethylene copolymer ingredient, and a propylene/ethylene copolymer as a compatibilizing agent ingredient are produced by three-stage polymerization using a Ziegler catalyst.
Recently, copolymers produced by that technique while regulating the intrinsic viscosities and MFRs of the ingredients were disclosed which comprise a crystalline polypropylene ingredient, a copolymer ingredient having a relatively low ethylene content as a compatibilizing agent, and a copolymer ingredient having a relatively high ethylene content (see patent documents 7 and 8). There is a statement in these patent documents that these copolymers are excellent also in low-temperature impact resistance. It is pointed out therein that for improving a balance among properties, to regulate only the ethylene content of each ingredient is insufficient and that intrinsic viscosity, i.e., molecular weight, and MFR should also be regulated so as to be in specific ranges. However, since these prior-art techniques employ a Ziegler catalyst, they intrinsically have the problems described above concerning a wide molecular-weight distribution or composition distribution. Furthermore, it has been pointed out that an ethylene/propylene copolymer containing at least 80 wt % propylene as a compatibilizing agent ingredient improves low-temperature impact resistance (see patent document 9).
An attempt has recently been made and proposed in which a propylene resin composition containing a propylene/ethylene copolymer as a compatibilizing agent ingredient and having an excellent balance among rigidity, heat resistance, and impact resistance is produced using a metallocene catalyst through polymerization in at least three stages as in the technique described above in which a Ziegler catalyst is used (see patent document 10). However, this document merely specifies each ingredient by showing wide ranges of the component proportion and ethylene content thereof, wide ranges of the intrinsic viscosity and MFR, etc., and it is difficult to consider the document to specifically disclose a process for producing the propylene resin composition having an excellent balance among rigidity, heat resistance, and impact resistance.
Moreover, an attempt has been made to obtain a propylene resin composition having an excellent balance among rigidity, impact resistance, and heat resistance by using an elastomer and an inorganic filler in combination with a block copolymer obtained by producing crystalline polypropylene and a propylene/ethylene copolymer by two-stage polymerization using a metallocene catalyst. However, since affinity between the crystalline polypropylene ingredient and the propylene/ethylene copolymer ingredient is relatively low as stated above, there has been room for improvement in balance among rigidity, heat resistance, and impact resistance. (See patent documents 11 and 12.)
There recently is a growing desire for an improvement in low-temperature (about −30° C.) impact resistance (cold resistance) besides ordinary-temperature impact resistance in frozen-food storage/packaging materials, industrial materials for use at low temperatures, etc. Although patent documents 3, 7, and 9, which were cited above, suggest an improvement in low-temperature impact resistance, this property is not always balanced with other properties. An improvement in this property is also desired.
As described above, propylene resins, which are exceedingly important industrial resin materials, have not been sufficiently improved in rigidity, heat resistance, and impact resistance while well balancing these. In addition, low-temperature impact resistance also is not always fully satisfactory. Improvements in these are presently expected.
Patent Document 1: JP-A-8-67783 (Abstract; Claims 1, 3, and 4; and paragraphs 0002 to 0004)
Patent Document 2: JP-A-4-337308 (Abstract)
Patent Document 3: JP-A-5-202152 (Abstract)
Patent Document 4: JP-A-6-172414 (Abstract)
Patent Document 5: JP-A-57-67611 (Claim 1)
Patent Document 6: JP-A-61-152442 (Claim (1); page 2, right lower column, lines 1-2; page 3, right upper column, line 5 from bottom to left lower column, line 2; and page 4, Example 1)
Patent Document 7: JP-A-2003-327642 (Abstract; Claim 1; and paragraph 0021)
Patent Document 8: JP-A-9-48831 (Abstract)
Patent Document 9: JP-T-2002-501555 (Abstract; Claim 1; and page 11, lines 9-10) (The term “JP-T” as used herein means a published Japanese translation of a PCT patent application.)
Patent Document 10: WO95/27741 (Abstract; Claim 1; and page 49, lines 1-3)
Patent Document 11: JP-A-10-1573 (Abstract)
Patent Document 12: JP-A-2003-147158 (Abstract)